1. Field of the Invention
The present invention relates to a driver circuit of an AMOLED (active matrix organic light emitting diode) with gamma correction, and more particularly, to a driver circuit of a current type AMOLED with gamma correction.
2. Description of the Related Art
Gamma correction is used to control the overall brightness of images shown on a display device. Images that are not properly corrected can look either bleached out or too dark. Generally, the optical characteristics of panel modules such as an LCD (liquid crystal display) and an electroluminescence panel have a nonlinear light transmission characteristic with respect to an applied voltage. Therefore, the drive circuit should drive the panel modules after the gamma correction is performed to correct the voltage in such a way as to match with the nonlinear light transmission characteristic of the panel modules. FIG. 1 illustrates a typical gamma correction curve with a gamma value of 2.2, in which both the x-axis (gray scale) and y-axis (relative luminance) are normalized. The applied voltage corresponding to a specific gray level is inputted into the panel module and then a relative luminance that is based on the gamma correction curve of FIG. 1 and corresponds to a specific light transmission is outputted.
FIG. 2 shows a conventional driver circuit 10 used in an LCD driver or in a voltage type AMOLED (active matrix organic light emitting diode) driver. The driver circuit 10 includes a gamma correction circuit 11, plural digital-to-analog converters DAC1-DACM, and plural operational amplifier buffers OPB. Each digital-to-analog convert DACi receives a digital code (i.e., a gray level or a pixel value) DGLi, and generates an analog driving signal ADSi in accordance with a reference voltage Vi generated from the gamma correction circuit 11. The analog driving signal ADSi is sent to drive the LCD panel or the voltage type AMOLED panel through the corresponding operational amplifier buffer OPB. The gamma correction circuit 11 includes plural resistors Ri connected in series between a high-potential power supply VH and a low-potential power supply VL, which is also called an R-string. It is easy to perform the gamma correction by the R-string configuration. The proper reference voltages Vi can be obtained by merely adjusting the resistances of the resistors Ri of the R-string to produce the desired gamma correction curve shown in FIG. 1 or the like. However, for a current type AMOLED driver, it is not so easy to achieve the desire gamma correction curve by the R-string.
FIG. 3 shows a conventional driver circuit 20 used in a current type AMOLED driver with a four-bit resolution. The driver circuit 20 includes plural groups of current mirrors. Each group of current mirrors includes four PMOS transistors and four corresponding switches b3-b0. For the current type AMOLED, the luminance thereof is proportional to the driving current flowing to the current type AMOLED. The arrangement of states (ON or OFF) of the four switches b3-b0 presents 16 different states to provide 16 levels of current to drive an AMOLED circuit. The switches b3 and b0 correspond to the MSB (most significant bit) and the LSB (least significant bit) of a digital input gray code, respectively. Therefore, the magnitude of the driving current is linearly dependent upon the pixel value. The driver circuit 20 can not achieve a non-linear gamma curve shown in FIG. 1.
FIGS. 4(a) and 4(b) show two relationship curves C1 and C2 (equivalent to gamma correction curves) between the driving current and the gray scale (six-bit resolution, for example) of two conventional driver circuits 20 with a small current scale and a large current scale, respectively. Another approach like the conventional driver circuit 20 is proposed, which combines FIGS. 4(a) and 4(b) to form FIG. 4(c). That is, the relationship curve C3 in FIG. 4(c) is a superposed curve of the relationship curves C1 and C2. However, the relationship curve C3 still cannot fit the desired gamma correction curve of FIG. 1. Therefore, this approach also fails to perform the gamma correction, and it is necessary to develop a driver circuit with gamma correction to drive a current type AMOLED.